Based on the web site I think he was set up, but the girl.
"Meeting up with girls nearly half your age - and under 18, at that - is just creepy."
But you must be 18 or older to use the site .
The charge sexual assault is bit miss leading In this context. If he had just had sex with her that would be statutory rape. In most states Sexual assault is one level below rape . Even if he is found not guilty of sexual assault his real problem is he paid a minor for sex. That's state and federal time